Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOx storage catalyst storing NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, wherein cylinders are divided into a first cylinder group and a second cylinder group and an exhaust passage of the first cylinder group and an exhaust passage of the second cylinder group are merged and connected to a common NOx storage catalyst (see Japanese Patent Publication (A) No. 8-189388). In this internal combustion engine, usually in all cylinders, fuel is burned under a lean air-fuel ratio and the NOx generated at this time is stored in the NOx storage catalyst. On the other hand, if the NOx storage ability of the NOx storage catalyst approaches saturation, the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst is temporarily made rich and thereby NOx is released from the NOx storage catalyst and reduced.
In this regard, fuel and lubrication oil contain sulfur. Therefore, the exhaust gas contains SOx. This SOx is stored together with the NOx in the NOx storage catalyst. In this regard, this SOx is not released from the NOx storage catalyst by just making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOx stored in the NOx storage catalyst gradually increases. As a result, the amount of NOx able to be stored gradually ends up becoming reduced. Therefore, when the amount of SOx stored in the NOx storage catalyst increases, it is necessary to make the NOx storage catalyst release the SOx.
In this case, if the temperature of the NOx storage catalyst is raised to the SOx release temperature of substantially 600° C. or more and the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst is made rich, the NOx storage catalyst can be made to release the SOx. In this regard, in the above-mentioned internal combustion engine, when the air-fuel ratios of all cylinders of the first cylinder group are made rich and the air-fuel ratios of the cylinders of the second cylinder group are made lean, the large amount of unburned HC exhausted from the first cylinder group is oxidized at the NOx storage catalyst by the excess oxygen exhausted from the second cylinder group and the NOx storage catalyst is raised in temperature by the heat of oxidation reaction at this time. Therefore, in the above-mentioned internal combustion engine, when making the NOx storage catalyst release SOx by raising the temperature of the NOx storage catalyst, the air-fuel ratios of all cylinders of the first cylinder group are made rich and the air-fuel ratios of all cylinders of the second cylinder group are made lean.
However, in this internal combustion engine, the NOx storage catalyst is arranged in the exhaust passage considerably far from the cylinders, so the NOx storage catalyst is low in temperature and not activated in many cases. In this regard, when the temperature of the NOx storage catalyst is low and the NOx storage catalyst is not activated in this way, if making the air-fuel ratio of one cylinder group rich and making the air-fuel ratio of the other cylinder group lean to raise the temperature of the NOx storage catalyst, a large amount of unburned HC ends up passing straight through the NOx storage catalyst without being oxidized and therefore not only is the temperature raising action not sufficiently performed, but also the problem arises that a large amount of unburned HC is exhausted into the atmosphere.